Second large earthquake on Philippine Fault in four days

Ormoc City on the island of Leyte in the Philippines was rattled by its second large magnitude earthquake in four days. Both of these quakes occurred on the highly active Philippine Fault.

Last Thursday (July 6), there was a M=6.5 earthquake on the island of Leyte in the Philippines. That earthquake resulted in two fatalities, 349 injuries, and over 1,700 partially to severely-damaged homes, according to the National Disaster Risk Reduction and Management Council (NDRRMC). We blogged about that quake, and that article can be found here. Now, at 9:41 a.m. local time today (July 10), a M=5.9 aftershock shook the region, approximately 20 km southeast of the mainshock’s epicenter. While the country expects to see some damage from this aftershock, none has been reported so far.

This figure shows the USGS locations of the M=6.5 mainshock in the Philippines on 6 July 2017, and the M=5.9 aftershock on 10 July 2017. This map also shows the location of the Philippine Fault, on which both of the these earthquake likely occurred. (Faults from G-EVER)

The M=6.5 mainshock last Thursday occurred on the highly-active Philippine Fault, which extends for 1,200 km throughout the entire Philippine archipelago. This left-lateral strike-slip structure has been responsible for numerous, large, damaging historic earthquakes. While there is no focal mechanism from today’s earthquake, based on its location and depth (12.7 km) it too likely occurred on the Philippine Fault. This is assuming that the fault in this location dips at a similar angle (73°) to how it does at the location of the mainshock.

Based on coulomb stress modeling, which can be seen below, the M=6.5 earthquake which struck last Thursday resulted in increased stress in the city of Ormoc, which is home to over 200,000 people. It should be stressed that the model below assumes a rupture length of approximately 18-20 km at a depth of 7 km roughly between the M=6.5 mainshock and M=5.9 aftershock. Therefore, the results should only be viewed as preliminary. Furthermore, it should be pointed out that we see no clear correlation between the aftershocks and the zones or increased and decreased stress. Nonetheless, these results highlight the lobes of increased stress, which are where large aftershocks could occur. Such results are important, especially when major cities lie in areas of increased hazard. Should any more large aftershocks occur, we will update this post.

This Google Earth image shows a preliminary coulomb stress calculation for the M=6.5 earthquake in the Philippines on 6 July 2017. The red dots are the earthquakes following the mainshock. The coulomb stress calculation was made by assuming a rupture length of approximately 20 km at a depth of 7 km.